This invention is related to a hold-down system for use with a mechanically operated dock leveler. Dock levelers or, dock boards, have a ramp which is hingedly mounted at its rear edge to a loading dock or to supporting structure set forth in a pit. The front edge of the ramp has an extension lip which is hinged so that it rests on the bed of a carrier, such as a truck which is loaded and/or unloaded at the dock. The dock board is a bridge between the carrier and the dock. The art is replete with a myriad of different configurations of such dock levelers with various types of actuating mechanisms, safe legs or the like.
This invention is directed to a technique of holding down the dock leveler in a particular position yet allowing the device to "float" as the truck is loaded and unloaded and so that the truck bed assumes a different vertical height.
The deck assembly of a mechanically operated dock leveler is spring loaded so that when the hold-down device is released the deck will tend to raise under the spring bias. As the deck goes up a mechanism automatically extends the lip from a downward pendant position to an outwardly extending one. With the deck thus raised, that is pivoted upward, an operator then walks the unit down by walking onto the deck using his weight to lower the deck until the lip comes to rest on the truck bed. The hold-down device maintains the deck at an operative position with the lip in contact with the truck bed. Most mechanical dock leveler hold-down devices employ a ratchet and pawl mechanism to hold the dock leveler from rising once placed in an operative position together with a float spring to allow the leveler to follow the height of the truck as it moves up and down with varying loads.
An example of a rack and gear mechanism is illustrated in U.S. Pat. No. 4,847,935 which utilizes a brake band as a technique of stabilizing the unit in position. Such a configuration is an advance over the conventional ratchet and pawl design which has only a limited travel with the float spring and stores high forces making the pawl difficult to release.
Another technique is the use of a friction brake to allow unlimited float. An over-running clutch allows the hold-down to retract and the brake slips as the hold-down is forced to extend. The brake is easy to release, but without a float spring the brake slips every time the leveler moves upward. An example of such a frictional brake is illustrated in U.S. Pat. No. 4,126,909. A slightly different mechanism which employs a spring in series with the brake is illustrated in U.S. Pat. No. 3,137,017.
In such systems the brake is easy to release but without a full length float spring there is slippage every time the leveler moves upward. Since the truck height and therefore that of the dock leveler varies each time a heavily loaded vehicle such as a fork lift travels over it there will be repeated brake slip resulting in excessive wear of the mating surfaces.
Additionally, most existing friction brake hold-down devices are complex requiring a brake and drum, overrunning clutches, cables and reels, or a rack and pinion. Given that dock levelers operate in a harsh environment subject to extremes of weather and debris it is important that these devices be as simple and reliable in their operation.
Yet another problem with traditional hold-down devices without float control is that the dock-leveler will not fall back down to contact a trailer bed after it has risen and then return to its previous height. This sequence occurs when a heavy fork truck is driven into a trailer which has a soft suspension and is nearly empty. As the fork lift truck enters the trailer, the leveler and the back of the trailer are depressed based on the weight which is applied. As the fork lift truck then moves forward on the truck base to the front of the trailer the load on the rear suspension is decreased and the rear of the trailer tends to rise. This in turn forces the dock leveler to rise and the hold-down mechanism to extend.
When the fork truck then attempts to reverse its position and back out of the trailer the weight of the fork lift again depresses the rear of the trailer. However, the dock leveler has no float associated with it and thus tends to remain at the previous height. The extended lip tends to fall to meet the trailer bed. The fork lift truck is then presented with a very high angled ramp in which it must climb. If the height difference is too great the angle is steep and the fork lift truck will strike the folded lip damaging the fork lift truck or leveler or, in a worst case condition causing potential injury to the driver.
Thus, there exists within this technology a need for a hold-down device that provides for an increased degree of float as the vehicle is loaded and/unloaded yet at the same time provides a simplification in the reduction of elements such as the elimination of ratchets, pawls, unnecessary cables and reels. Moreover, in the context of devices which use brake mechanisms, a minimization of wear of the operational elements as a consequence of brake slippage is highly desirable.